Motor drive arrangements of the above type are often employed in AC electric drive applications, such as electric vehicles or locomotives. In some applications, the supply voltage is provided by a battery pack; in other applications, by an engine driven alternator and rectifier assembly. In either case, the source provides a DC supply voltage which the voltage source inverter (VSI) applies to the phase windings of the AC motor.
Various protection schemes, such as inverter fuses and source voltage control, have been developed to protect the switching devices of the inverter in the event of transient overcurrent or overvoltage conditions, and to minimize the corresponding source and motor torque transients. However, fuses must be reset to resume operation once a fault occurs, and source voltage controls are typically not fast enough to adequately protect the inverter switching devices.
According to another protection scheme, a high capacity thyristor, referred to herein as a crowbar device, is connected across the DC supply voltage and gated to a conductive state upon detection of an overvoltage or overcurrent condition. When the transient condition subsides, the thyristor is commutated and normal operation resumes. A typical crowbar arrangement in a locomotive traction drive is depicted in FIG. 1.
Referring to FIG. 1, the rectified output voltage of an engine driven alternator 10 is applied to the phase windings 12a, 12b and 12c of an AC traction motor 12 via filter capacitor Cf and the thyristors 14-24 of a voltage source inverter 25. Freewheeling diodes 26-36 parallel each of the thyristors 14-24 to protect the respective thyristors by circulating inductive motor currents at the commutation intervals. A crowbar thyristor 42 is connected in parallel with the inverter 25 and gated conductive to protect the inverter thyristors 14-24 during overvoltage and overcurrent conditions. The element Lm designates lead inductance between the alternator and the crowbar thyristor 42.
When the crowbar thyristor 42 is gated into conduction in response to a detected overvoltage or overcurrent condition, the inverter thyristors are commutated and the traction motor 12 operates as a generator due to the magnetic flux stored in its windings. In this mode, the motor windings supply an unbalanced transient current Im to the crowbar thyristor 42 via a pair of freewheeling diodes. In FIG. 1, the current Im is supplied via freewheeling diodes 26 and 36, indicating that the inverter thyristors 16 and 22 had been conductive at the time of the fault. A similar condition occurs in the alternator 10, which also supplies a rectified unbalanced transient current Ia to the crowbar thyristor 42. Additional crowbar current is supplied by the filter capacitor Cf.
While the crowbar thyristor 42 may be sized to withstand the combined motor, alternator and capacitor fault currents, the transient currents Im and Ia each produce transient torques and magnetic forces in the respective machines 10, 12. These torques and forces severely stress various internal elements of the machines 10, 12, possibly contributing to premature failure. Particularly vulnerable machine elements include shafts, stator end-windings and rotor field poles.